Polyphenylene Nanostructures - Cluster for Molecular Chemistry

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1752 Chemical Reviews, 1999, Vol. 99, No. 7 Berresheim et al. Scheme 6 Scheme 7 by condensation of perfectly end-functionalized poly- (2,5-diheptyl-para-phenylenes) (35) (PPP) with polystyrene (PS) (36) or poly(ethylene oxide) (PEO) (37) was reported. 90 In this case, PPP chains with 7-10 repeat units were used. Employing the Suzuki coupling of aromatic monomers 69,70 in preparing the PPP block has the advantage of ensuring an aromatic block with far fewer defects. B. Synthesis of Bridged Polyphenylenes: Step-Ladder and Ladder Polymers Although alkyl substituents in the 2- and 5- positions lead to a considerable improvement in the solubility of poly(para-phenylene)s, they also have an important disadvantage with respect to the electronic properties of PPPs. As a result of the steric demands of the substituents, the dihedral angle between the individual polymer chains increases from 23° for an unsubstituted PPP 91 to 60-80° for dialkyl-substituted PPP derivatives, 92 which is coupled with a drastic reduction of the electronic conjugation along the main PPP chain. In Chapter VI the importance of alkyl groups in oligo- and polyphenylenes for the realization of supramolecular order will be shown. However, an important target in the synthesis of new PPP derivatives is the preparation of soluble, structurally defined phenylenes that are easy to process and that show, in comparison with the parent compound 12a, intact or even increased π-electron conjugation. A key synthetic step toward the realization of this target is the connection of solubilizing substituents with simultaneous conversion of neighboring paraphenylene units into planar or only slightly twisted conformations. This synthetic concept was realized by Yoshino et al. 93 through the iron(III) chloride catalyzed oxidative polymerization of 9,9-di-n-hexylfluorene (40, Scheme 8) under Kovacic conditions. 94,95 The di-n-hexylmethylene bridge provides adequate solubility in the poly(9,9-di-n-hexylfluorene-2,7-diyl) (41) and simultaneously forces the planar ordering of neighboring phenylene building blocks, which results in a 50 nm bathochromic shift of the longwavelength absorption maxima in comparison with that of unsubstituted PPP (12a). However, the structural defects, which appear as the result of a 2,7coupling during the aryl-aryl coupling, and the low degree of polymerization of maximum 20 aromatic rings remain a disadvantage.
Polyphenylene Nanostructures Chemical Reviews, 1999, Vol. 99, No. 7 1753 Scheme 8 In 1993, Yamamoto et al. described a simple approach to a further partially bridged PPP derivative with the synthesis of poly(9,10-dihydrophenanthrene-2,7-diyl) (43, Scheme 8). Starting from 2,7dibromo-9,10-dihydrophenanthrene (42), they used the advantage of the efficient and highly selective nickel(0)-catalyzed homo-aryl-aryl coupling. 96 As a consequence of the low solubilizing effect of the ethanediyl bridges, only the oligomer fraction (Mn < 1000) of the resulting polymer 43 was soluble while the polymeric product precipitated from the reaction solution. On the basis of these results, the nickel(0)catalyzed aryl-aryl coupling of suitable alkyl-substituted dibromodihydrophenanthrenes or dibromotetrahydropyrenes for the preparation of soluble stepladder polymers was an obvious choice. In 1995, Müllen et al. realized this synthetic concept, 21,97 by subjecting 2,7-dibromo-4,9-di-n-alkyl-4,5,9,10-tetrahydropyrene (44) as a monomeric building block to the conditions of the Yamamoto coupling (Scheme 9). Scheme 9 The poly(4,9-di-n-alkyl-4,5,9,10-tetrahydropyrene- 2,7-diyl)s (45) obtained were completely soluble and possessed relatively high molecular weights (Mn up to 20 000), corresponding to a chain length of approximately 46 repeat units. In subsequent work they also achieved separation of the diastereomers used as monomers by fractional crystallization and chromatography, whereby stereoregular poly(tetrahydropyrene)s 45 (cis- ortrans-polymers) became accessible. The longest-wavelength absorption maximum λmax of the poly(tetrahydropyrene)s 45 is 385 nm and is thus almost identical with the value determined by Yoshino et al. for the step-ladder polyfluorene 41. 93 The poly(tetrahydropyrene)s 45 gave an intense blue photoluminescence in solution with a small Stokes shift (40 nm, λmax emission: 425 nm) and also found was a slight bathochromic shift in the solid state as a result of aggregation. Thus, experiments were conceived to examine the suitability of the poly- (tetrahydropyrendiyl)s 45 as emitters in the production of light-emitting diodes. In this respect, the principal use of suitably modified PPP derivatives was shown to be blue emitters. 21,97 A further step in the concept of bridged PPP derivatives was the transition from step-ladder to ladder polymers. Twisting along the main polymer chain can be minimized by complete bridging of all phenylene rings with each of the respective neighboring monomer units. Thus, the maximum conjugation of π-electrons resulting from the planarization leads to an optimization of the chromophoric properties of poly(para-phenylene) compounds. An example of a ladder polymer synthesis of this type is the synthetic sequence published in 1991 by Scherf and Müllen, 98 which started from a dialkyl-substituted phenylenediboronic acid 46 and an aromatic dibromodiketone 47 and led, via the open-chain, single-stranded precursor PPP 48 in a multistep sequence, to the double-stranded ladder polymer 50, Scheme 10. With Scheme 10 the synthesis of the well-defined ladder polymer 50, it was possible to show that the polymer-analogous ring closure by intramolecular Friedel-Crafts alky-
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